The present invention relates to linear output regulator circuits and, more particularly, to linear output regulator circuits having two control loops.
Linear regulators typically include an error amplifier in a feedback loop for comparing an output level to a reference level. Due to the gain provided by the error amplifier, the feedback loop can cause the linear regulator to become unstable or oscillate under certain conditions. To ensure stability, the total gain of the feedback loop must be less than one (unity) when the signal response of the linear regulator is 360xc2x0 out of phase. Many linear regulators employ an external capacitor connected in parallel with the load to ensure stability. This external capacitor provides a pole that causes an additional 20 dB/decade of roll off in the feedback loop so that the gain of the loop will be less than one before the phase response has shifted 360xc2x0.
The value of the external capacitor is chosen to introduce a pole so that the gain of the feedback loop is less than one when the phase response shifts 360xc2x0. In addition, this capacitance helps transient response by providing fast current response to changes in the output current of the linear regulator. However, the linear regulators can still be susceptible to oscillation if the value of the external capacitor does not stay within a relatively tight tolerance. Therefore, it is desirable to increase the phase margin in linear regulators without increasing the external capacitance.
In accordance with aspects of the present invention, a linear regulator with two control loops is provided. In one aspect of the present invention, the linear regulator includes an outer control loop, an inner control loop, an output energy storage device, and a power modulator. The outer control loop and the inner control loop monitor different control variables where conversion from one control variable to the other provides a positive phase shift. For example, in one embodiment, the outer control loop monitors voltage and the inner control loop monitors current, where conversion from voltage to current monitoring provides a positive phase shift.
In operation, the outer control loop monitors a first control variable of the output signal of the linear regulator by comparing the level of the first control variable of the output signal to a reference level. The first control loop then generates an error signal related to the difference between the reference level and the first control variable level. An inner control loop monitors a second control variable of the output signal by comparing the level of the second control variable to the level of the error signal of the outer control loop. That is, the error signal of the outer control loop serves as the reference value of the inner control loop. Also, the error signal of the inner control loop then serves as the control signal of the power modulator, which provides the output signal to a load.
An energy storage device, e.g., a capacitor, is connected in parallel to the load to provide a compensating pole and improve transient response of the linear regulator. The outer control loop regulates the output signal by regulating the first control variable. In this aspect, the first control variable is selected to be the more slowly varying control variable between the first and second control variables. Similarly, the inner control loop regulates the output signal by regulating the second control variable. Also, because the conversion from the first control variable to the second control variable provides a positive phase shift, the use of an inner control loop provides an additional degree of phase margin, thereby improving stability.
In another aspect of the present invention, a multi-loop linear voltage regulator is provided. The multi-loop linear voltage regulator includes a voltage error amplifier, a current error amplifier, a pass transistor, a voltage to current converter, and an output capacitor. The pass transistor is connected to a power source and provides an output signal to the load. The voltage-to-current converter monitors the voltage level of the output signal and outputs a current having a level that is dependent on the output voltage level. The voltage error amplifier compares the output voltage to a reference voltage. The voltage error amplifier outputs an error current signal having a level that depends on the difference between the output voltage and the reference voltage. The current error amplifier compares the error current signal to the output current of the voltage-to-current converter and provides a control signal to the pass transistor having a level that is dependent on this difference. The voltage error amplifier provides a relatively slow voltage regulation, with the output capacitor providing stability and improved transient response. The current error amplifier provides a fast current regulation while gaining about 90xc2x0 of phase margin, thereby improving both stability and small signal response.